The present invention relates to methods for controlling the quality of an industrial process, comprising the steps of:
making available one or more reference signals relating to the industrial process
acquiring one or more real signals indicating the quality of said industrial process,
comparing said one or more reference signals to said one or more real signals to identify defects of said industrial process
Monitoring defects in industrial processes is assuming a growing economic importance due to its impact in the analysis of the quality of industrial products. The ability to obtain an assessment of the quality of the industrial process in line and automatically has many advantages, both in economic terms and in terms of process velocity. Therefore, the desirable characteristics of the system are:                on line and real time processing;        ability to recognise the main production defects with accuracy.        
Currently, the problem of recognising the quality of an industrial process, and thus of identifying any defects, takes place through an off-line inspection by experts, or with automatic methods which, through sensors, identify only some of the aforementioned defects, in a manner that is not satisfactory and that is also sensitive to the different settings of the machine.
Methods and systems for controlling the quality of industrial processes are known, for instance applied to the on-line monitoring of the laser welding process, in particular in the case of metal sheet welding. The controlling system is able to assess the presence of porosities in the welded area or, in the case of butt-weeded thin metal sheets, the presence of defects due to the superposition or to the disjunction of the metal sheets.
Said used systems base quality control on a comparison between the signals obtained during the process and one or more predetermined reference signals, indicative of a high quality weld. Said reference signals, usually in a variable number between two and ten, are predetermined starting from multiple samples of high quality welds. Obviously, this way of proceeding implies the presence of an experienced operator able to certify the quality of the weld at the moment of the creation of the reference signals, entails time wastage and at times also material wastage (which is used to obtain the samples needed to obtain the reference signals). In some cases, reference signals indicating a defective weld are also arranged, and this entails additional problems and difficulties.
The European patent application EP-A-1275464 in the name of the present Applicant teaches to divide into blocks the signal acquired by means of a photodiode which collects the radiation emitted by a weld point, calculating the mean of the signal in each sampled block and taking in account the blocks whose value is lower than or equal to the offset of the photodiode to be indicative of the presence of a defect. Said method eliminates the need for the reference, but it allows for a very approximate detection of defects.